In an automatic machine tool or machine center, a tool magazine with a number of different tools contained therein is provided for storing all tools needed for a specific machining process. These tools are interchanged automatically by a control unit or, in a numerical control (NC) or computerized numerical control (CNC) machine tool, a microprocessor via a tool changing mechanism which is herein referred to as "tool magazine mechanism."
Currently, most of the commercially available automatic machine tools or machine centers adopt the so-called Geneva mechanism to rotate the tool holding disk of the machine tool for changing the tools. In FIG. 1 of the drawings, a Geneva tool magazine mechanism is illustrated. The Geneva mechanism comprises a drive member 13 which cooperates with a follower member 22 on which a number of radially-extending index slots 21 are formed. A driving pin 11 is mounted on the drive member 13 so that when the drive member 13 rotates, the pin 11 will enter one of the index slots 21 to have the follower member 22 driven in an indexing manner.
The motion of the follower member of the Geneva mechanism is illustrated in FIG. 2 of the attached drawings. The motion of the Geneva mechanism is a function of the index number of the follower member, which is denoted by "N" in the drawing. As illustrated, the smaller the index number N, the steeper the angular acceleration curve. The index number N is an indication of the number of drive member revolutions necessary for rotating the follower member a full turn. A small index number represents that each turn of the drive member rotates the follower member through a large angular displacement. In other words, the follower member is rotated faster during the index period for a smaller index number.
It is found from the angular acceleration curves shown in FIG. 2 that an infinite jerk, which is the time derivation of acceleration, will be induced. The abrupt change in motion occurring when the driving pin 11 enters the index slots 21 is the major cause for the infinite jerk. Such an infinite jerk will lead to noise, vibration, impact, wearing and non-smooth motion eventually damaging the tool magazine mechanism.